1. Field of the Invention
This patent application relates to a reduced lead brass alloy that is both machinable and corrosion resistant. More particularly, an .alpha.+.beta. brass containing bismuth and phosphorous, as well as, optionally tin is claimed. Within specific composition limits, these additions improve machinability and corrosion resistance without a detrimental effect on machinability.
2. Background
Free machining copper alloys contain lead or other additions to facilitate chip formation and the removal of metal in response to mechanical deformation caused by penetration of a cutting tool.
The addition to the alloy is selected to be insoluble in the copper based matrix. As the alloy is cast and processed, the addition collects both at boundaries between crystalline grains and within the grains. The addition improves machinability by enhancing chip fracture and by providing lubricity to minimize cutting force and tool wear.
Brass, a copper-zinc alloy, is made more machinable by the addition of lead. One example of a leaded brass is alloy C360 (nominal composition, 61.5% copper, 35.5% zinc and 3% lead). Throughout this patent application, all alloy compositions are in weight percent unless otherwise indicated. Alloy C360 has high machinability and acceptable corrosion resistance. Alloy C360 is commonly used in environments where exposure to water is likely. Typical applications include plumbing fixtures for potable water.
The ingestion of lead is harmful to humans, particularly children with developing neural systems. To reduce the risk of exposure, lead has been removed from the pigments of paints. It has now been proposed in the United States Senate to reduce the concentration of lead in plumbing fittings and fixtures to a concentration of less than 2% lead by dry weight. There is, accordingly, a need to develop machinable copper alloys, particularly brasses, which meet the reduced lead target.
One such alloy is disclosed in U.S. Pat. No. 4,879,094 to Rushton. The patent discloses a cast copper alloy which is substantially lead free. The alloy contains, by weight, 1.5-7% bismuth, 5-15% zinc, 1-12% tin and the balance copper. The alloy is free machining and suitable for use with potable water. However, the alloy must be cast and is not wrought.
A wrought alloy is desirable since the alloy may be extruded or otherwise mechanically formed into shape. It is not necessary to cast objects to a near net shape. Wrought alloy feed stock is more amenable to high speed manufacturing techniques and generally has lower associated fabrication costs than cast alloys.
Another free machining brass is disclosed in Japanese Patent Application 54-135618. The publication discloses a copper alloy having 0.5-1.5% bismuth, 58-65% copper and the balance zinc. The replacement of lead with bismuth at levels up to 1.5% will not provide an alloy having machinability equivalent to that of alloy C360.
Wrought bismuth containing copper alloys are disclosed in U.S. Pat. No. 5,167,726 to LoIacono et al. The alloy contains in excess of 0.1% phosphorous, 0.25% indium and/or 0.5% tin to reduce embrittlement of the copper alloy caused by the bismuth addition. As the phosphorous content is increased, phosphides form in the alloy. These phosphides are significantly harder than the copper base alloy and cause excessive wear to machining tools.
Phosphorous is also disclosed to increase the hot tearing of a bismuth containing brass casting in an article by Oya et al. entitled "Low Melting Point Inclusions and Hot Tearing in Brass Castings". An article by Price entitled "Bismuth--Its Effect on the Hot-working and Cold-working Properties of Alpha and Alpha-beta Brasses" discloses that the addition of 0.01%-0.02% phosphorous eliminated fire cracking in a 70% copper-30% zinc alloy (.alpha. brass) containing 1% bismuth.
Fire-cracking is the formation of cracks when a cold worked alloy is heated rapidly to above the recrystallization temperature. According to the Price article, the presence of internal stresses due to working the alloy followed by the sudden application of heat causes fire-cracking.
The presence of .beta. phase in a bismuth containing brass greatly improves the hot working capability of the alloy. However, when the alloy is used for water transport or water storage, the .beta. phase is susceptible to corrosion. There exists, therefore, a need for a bismuth containing copper alloy that is resistant to fire-cracking and corrosion and also has good machinability.